August 2021
Intermediate to advanced
450 pages
4h 46m
Chinese
Content preview from 面向对象的思考过程（原书第5版）
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9.5.2 可选的联合
当处理联合时最重要的问题之一是确保应用程序设计用于检查可选的联合。即代码必须检查该联合是否为null。
假设之前的示例代码中每名雇员有一个配偶。然而，如果雇员没有结婚，代码则会出问题（如图9.8）。如果代码期望有一个配偶，结果会出问题，并导致系统处于不稳定的状态。所以代码必须检查null情况，并且必须作为有效的情况进行处理。
图9.8 检查所有可选的联合关系
例如，如果雇员没配偶，代码不应该试图调用配偶的行为，这会导致应用程序崩溃。代码必须能够处理Employee对象没有配偶的情况。
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